(1) Field of the Invention
The present invention relates to an apparatus and method and in particular but not exclusively for the acquisition of signals.
(2) Description of Related Art
In an example of a global navigation satellite system satellites orbiting the earth in known orbit paths with accurately known positions are used. These satellites transmit signals which can be received by a receiver on earth. Using signals received from four or more satellites, the receiver is able to determine its position using trigonometry. The signals transmitted by the satellite comprise pseudo-random codes. The accuracy of the determination of position is dependent on factors such as the repetition rate of the code, the components of the receiver and atmospheric factors.
GALILEO is a European initiative for a global navigation satellite system which provides a global positioning service. It has been proposed that GALILEO be interoperable with the global positioning system GPS and GLONASS, the two other global satellite navigation systems. It should be appreciated that the term GNSS is used in this document to refer to any of these global positioning systems.
GALILEO currently has a system of thirty satellites, twenty-seven operational satellites with three operational in-orbit spares. The proposed frequency spectrum for GALILEO has two L-bands. The lower L-band, referred to as E5a and E5b, operate in the region of 1164 MHz to 1214 MHz. There is also an upper L-band operating from 1559 MHz to 1591 MHz.
In GPS and GALILEO, signals are broadcast from satellites which include the pseudo random codes which are processed at a receiver to determine position data. The processing involves first determining the relative offset of the received codes with locally generated versions of the codes (acquisition) and then determining the position once the relative offset is determined (tracking). Both acquisition and tracking involve correlating received signals with a locally generated version of the pseudo random codes over an integration period.
In spread spectrum systems, acquisition may be difficult because it is two dimensional (frequency and time). A further difficulty is that because the signals are much weaker inside as compared to outside, it is much more difficult to acquire signals indoors. In particular, the indoor operation of GNSS requires the reception of signals attenuated by at least 20 dB from the outdoor equivalents.
Acquisition is carried out by a trial and error searching of cells corresponding to a frequency and phase range. The number of cells in the time domain is for example 4092. The number of cells in the frequency domain increases with a drop in signal strength. This however may be reduced with use of a temperature controlled crystal oscillator TCXO. The time required to search a cell may increase one hundred fold from outdoors to indoors. For example for indoors, each cell may take 100 milliseconds because of the weaker signal strength. This results in a greatly increased search time for indoor receivers.
This problem may be addressed by using parallelism in the frequency domain, for example sixteen fast Fourier transform channels or by parallelism in the time domain, using parallel correlators. To achieve parallelism may require faster clocks and/or more hardware which may be disadvantageous. Additionally, more hardware and/or faster clocks may require increased power.
In any event, one limit is the stability of the reference clock which may prevent bandwidth reduction to the degree required for indoor sensitivity.
As already mentioned the indoor signals can be attenuated by at least 20 dB from their outdoor equivalents. To increase the sensitivity by 20 dB for the indoor signals means integrating for a hundred times longer. However, this may be difficult to achieve because as the coherent integration period is extended, the bandwidth of the channel is narrowed. This in turn requires many more searches to be carried out and eventually the stability of the reference oscillator becomes a limiting factor as a signal appears to wander from one frequency to another, even before acquisition is completed. This results in a spreading of the energy, preventing further gain.
In addition, the modulation method used may provide a limit on the integration time.
Thus there may be problems in performing integration with such signals. The integration time may be limited by the accuracy of a local clock and the frequency shifts caused by relative motion of the satellite and receiver.